Employing morphologic, enzymologic and karyologic parameters, the process of oncogenesis in the bladders of rats exposed to the carcinogen, N-(4-(5-Nitro-2-furyl)-2- thiazolyl) formamide will be studied on a sequential basis; in conjunction with this, like parameters will be investigated in human bladder neoplasms and attendant situations--viz. papillomas, carcinomas of different grades, atypical urothelium neighboring frank tumors and urothelium followed in patients whose tumor has been locally resected. Because distinctive alterations occur in the LDH isozyme pattern and in the chromosomal composition of tissues with the onset of malignant transformation and because it is likely that a considerable proportion of human bladder cancers are carcinogen-induced, these experimental and clinical situations may validly be compared and contrasted. It is expected that information will be forthcoming which will permit the more precise diagnosis of doubtfully malignant bladder tumors, the more exact prognosis of the course that may be followed by residual urothelium when localized neoplasms have been resected, and, consequently, the more refined treatment of individual patients with bladder cancer.